1. Field of the Invention
The present invention relates to a method of coating a coating solution such as a photoresist to the surface of a substrate such as a glass substrate, a semiconductor wafer, or the like.
2. Description of the Related Art
Photoresists for forming color filters are uniformly coated on the surface of a glass substrate for use in liquid-crystal display devices, and various coating solutions are uniformly coated on the surface of a semiconductor wafer. It has been customary to apply a uniform coating of such a coating solution by dropping the coating solution onto the center of the substrate and then rotating, with a spinner, the substrate to spread the dropped coating solution uniformly over the entire surface of the substrate under centrifugal forces.
According to the above coating process using a spinner, a large amount of coating solution tends to be scattered off the substrate because it is necessary to spread the coating solution dropped onto the center of the substrate uniformly outwardly to the peripheral edge of the substrate. As a result, the coating solution is consumed in a large quantity.
Use of a slit nozzle for minimizing the amount of coating solution consumed is disclosed. in Japanese laid-open patent publications Nos. 63-156320, 4-332116, and 6-151296.
According to the processes disclosed in Japanese laid-open patent publications Nos. 63-156320 and 4-332116, while the slit nozzle positioned above a substrate is being translated parallel to the substrate, a coating solution is dropped from the slit nozzle onto the substrate. After the coating solution is deposited to a somewhat uniform thickness on the surface of the substrate, the substrate is rotated to uniformize the thickness of the coating solution thereon. However, since the coating solution is simply dropped on the surface of the substrate, the coating solution cannot completely uniformly be deposited because of the surface tension of the dropped coating solution. Therefore, it is necessary to apply a large amount of coating solution in order to form a uniform film of coating solution on the substrate. Another problem is that because the substrate is rotated in an atmosphere vented to the ambient air, the solvent in the coating solution tends to be evaporated quickly, making the coating solution more viscous soon. Therefore, it soon becomes difficult to spread the coating solution uniformly over the entire surface of the substrate.
According to the process revealed in Japanese laid-open patent publication No. 6-151296, the tip end of the slit nozzle is used to uniformize the thickness of a film of coating solution on the surface of a substrate. However, the slit nozzle is liable to be smeared with coating solution as its tip end contacts the coating solution. Further, the coated film tends to become irregular in thickness when the slit nozzle is lifted off the coating solution after the coating solution is deposited on the substrate.
It is therefore an object of the present invention to provide a method of coating a coating solution quickly to a thin uniform thickness on the surface of a substrate.
According to an aspect of the present invention, there is provided a method of coating a solution on a surface of a rectangular substrate, comprising the steps of positioning a slit nozzle above the rectangular substrate, ejecting the solution from the slit nozzle toward the surface of the rectangular substrate while reducing or minimizing the surface tension of the solution, translating the slit nozzle parallel to the rectangular substrate to coat the solution on substantially the entire surface of the rectangular substrate while the solution is being ejected from the slit nozzle, and thereafter rotating the rectangular substrate to spread the solution uniformly over the surface of rectangular substrate.
According to another aspect of the present invention, there is also provided a method of coating a solution on a surface of a circular substrate, comprising the steps of positioning a slit nozzle above the circular substrate, ejecting the solution from the slit nozzle toward the surface of the circular substrate while reducing or minimizing the surface tension of the solution, rotating the circular substrate at a first speed to coat the solution on substantially the entire surface of the circular substrate while the solution is being ejected from the slit nozzle, and thereafter rotating the circular substrate at a second speed greater than said first speed to spread the solution uniformly over the surface of circular substrate.
The surface tension of the solution may be reduced or minimized by reducing the distance between a lower end of the slit nozzle and the surface of the substrate and pressurizing the solution. Specifically, when the solution has a viscosity of at most 10 cP, it may be pressurized under a pressure ranging from 0.5 kg/cm2 to 10 kg/cm2, and the distance between the lower end of the slit nozzle and the surface of the substrate may be at most 10 mm.
According to still another aspect of the present invention, there is also provided a method of coating a solution on a surface of a rectangular or circular substrate, comprising the steps of setting the rectangular or circular substrate in a rotary cup, roughly coating the solution on substantially the entire surface of the rectangular or circular substrate, and thereafter rotating said rotary cup to spread the coated solution uniformly over the surface of the rectangular or circular substrate under centrifugal forces. It is preferable to close an upper opening of the rotary cup before the rotary cup is rotated.
According to yet still another aspect of the present invention, there is also provided a method of coating a solution on a surface of a rectangular or circular substrate, comprising the steps of roughly coating the solution on substantially the entire surface of the rectangular or circular substrate, immediately thereafter setting the rectangular or circular substrate in a rotary cup, and then rotating said rotary cup to spread the coated solution uniformly over the surface of the rectangular or circular substrate under centrifugal forces. It is preferable to close an upper opening of the rotary cup before the rotary cup is rotated.
The solution may be roughly coated on substantially the entire surface of the rectangular or circular substrate by a slit nozzle, a jet nozzle, a roll coater, or a bar coater.
Since the distance between the lower end of the slit nozzle and the surface of the substrate is reduced and the solution is pressurized, according to some of the aformentioned aspects of the invention, it is possible to apply, to the solution, forces tending to cancel out or minimize effects of the surface tension of the solution when it is brought into contact with the surface of the substrate.
After the rectangular or circular substrate is set in the rotary cup, the upper opening of the rotary cup is preferably closed and then the rotary cup is rotated to spread the solution uniformly over the rectangular or circular substrate under centrifugal forces according to other aformentioned aspects of the invention. Therefore, it is possible to spread the solution over the substrate in an atmosphere which makes it difficult to dry the solution.
The above and further objects, details and advantages of the present invention will become apparent from the following detailed description of preferred embodiments thereof, when read in conjunction with the accompanying drawings.